To get in line with the global trend of environmental protection, and eco-friendly regulations, electronic product manufacturers nowadays are developing, and manufacturing halogen-free electronic products. Advanced countries, and electronic manufacturing giants set forth schedules of launching mass production of halogen-free electronic products. As a result of the promulgation of the Restriction of Hazardous Substances (RoHS) by the European Union, hazardous substances, such as lead, cadmium, mercury, hexavalent chromium, poly-brominated biphenyl (PBB), and poly-brominated diphenyl ether (PBDE), are strictly prohibited from being used in manufacturing electronic products or their parts, and components. A printed circuit board (PCB) is an indispensable, and fundamental basis of the semiconductor industry, and electronic industry; hence, printed circuit boards bore the brunt of international halogen-free regulations when international organizations set forth strict requirements of the halogen content of printed circuit boards. For example, the International Electrotechnical Commission (IEC) 6 1249-2-21 requires that bromide content, and chloride content shall be less than 900 ppm, and the total halogen content shall be less than 1500 ppm. The Japan Electronics Packaging, and Circuits Association (JPCA) requires that both bromide content, and chloride content shall be less than 900 ppm. To enforce its green policies, Greenpeace calls on manufacturers worldwide to get rid of polyvinyl chloride (PVC), and brominated flame retardants (BFRs) from their electronic products in order to conform with the lead-free, and halogen-free requirements of green electronics. Hence, the industrial sector nowadays is interested in rendering related materials halogen-free, and sees this technique as one of its key research topics.
Electronic products nowadays have the trend toward compactness, and high-frequency transmission; hence, circuit boards nowadays typically feature a high-density layout, and increasingly strict material requirements. To mount high-frequency electronic components on a circuit board, it is necessary that the substrate of the circuit board is made of a material of a low dielectric constant (Dk), and dielectric dissipation factor (Df) in order to maintain the transmission speed, and the integrity of a signal transmitted. To allow the electronic components to operate well at a high temperature, and a high-humidity environment, it is necessary for the circuit board to be heat resistant, fire resistant, and of low hygroscopicity. Epoxy resin is adhesive, heat resistant, and malleable, and thus is widely applicable to encapsulants, and copper clad laminates (CCL) of electronic components, and machinery. From the perspective of fire prevention, and safety, any applicable material is required to be capable of flame retardation. In general, epoxy resin is incapable of flame retardation, and thus epoxy resin has to acquire flame retardation capability by including a flame retardant therein. For example, a halogen, especially bromine, is included in epoxy resin to bring about flame retardation capability of epoxy resin, and enhance the reactivity of the epoxy group.
A conventional circuit board manufacturing method, such as a conventional method of manufacturing a copper-clad substrate (also known as copper clad laminate, CCL), involves heating, and combining a reinforcement material (such as a glass fiber fabric), and a thermosetting resin composition made of an epoxy resin, and a curing agent to form a prepreg, and then laminating the prepreg, and the upper and lower copper foils together at a high temperature, and a high pressure. The prior art usually teaches using a thermosetting resin composed of an epoxy resin, and a hydroxyl-containing phenol novolac resin curing agent. However, phenol novolac resin exhibits so high a reactivity that it is flawed with an overly short expiration period when used as latex paint, not to mention that it is difficult to store and intolerant to heat.
U.S. Pat. No. 6,509,414 and U.S. Pat. No. 7,897,258 disclose a resin composition comprising a styrene-maleic anhydride (SMA) copolymer functioning as a co-crosslinking agent, an epoxy resin, and a co-crosslinking agent. The resin composition is characterized in that the co-crosslinking agent comprises an optionally brominated bisphenol A (BPA), an optionally brominated bisphenol A diglycidyl ether (BPADGE), or a mixture thereof, wherein the resin composition is free from an allyl network forming compound. The co-crosslinking agent is, for example, tetrabromobisphenol A (TBBPA), tetrabromobisphenol A diglycidyl ether (TBBPADGE), or a mixture thereof, such that the resin composition exhibits a high degree of thermal stability and high glass transition temperature (Tg). However, with the bisphenol A bromide functioning as the co-crosslinking agent for styrene-maleic anhydride (SMA) copolymer, the resin composition is flawed disadvantageously with an overly low glass transition temperature Tg.
Accordingly, it is important for printed circuit board material suppliers to develop halogen-free materials of a high glass transition temperature and high heat resistance, and apply the materials to printed circuit board manufacturing.